1. Field of the Invention
This invention relates to the field of radiography, and more particularly to a fourth generation Computed Tomography (CT) Scanner having an optimized scanner geometry.
2. Description of the Prior Art
Computed tomography processes are a modern technique developed to provide a non-invasive means for revealing internal organs and tissues of a human body in cross-section for the purpose of diagnosing the condition of the particular cross-section of the body being examined. Basically, computed tomographic techniques utilize a series of x-ray projections or views made from different angles, which are taken axially through a "slice" or thin cross-sectional area of a patient. The X-rays generated by a radiographic source, or in some instances Gamma Rays from a radioactive source, are transmitted transversely through a section of the patient's anatomy and are detected by a high-efficiency radiation detectors. Each projection or view of the cross-sectional "slice" consists of a series of ray paths or samples through the section with differences in the ray samples arising from attenuation differences in the slice or differences in path length through the slice. The different angular projections of the subject with each angular projection, comprised of a series of discrete ray samples provides the input by which x-ray or gamma ray attenuation coefficients can be calculated in a computer and the image of the cross-section of the patient's anatomical variations reconstructed. Thus, a computed tomography scanner obtains, by mathematical reconstruction, a transverse sectional image from transmitted radiation projection data, as is well known.
In the past several years, advances in computed tomography techniques have resulted in several generations of scanners. In the first scanner commercially marketed, the series of ray samples comprising a given projection was obtained by a translational movement of the x-ray source and a detector. The source and detector were stopped and rotated and the sequence repeated to obtain the different angular projections. In a 2nd generation scanner the data gathering process is much the same except several detectors were employed allowing for greater angular increments per rotation and fewer rotations resulting in shorter scan times. In 3rd generation geometries the x-ray source and detector array pivot around a common point in a single rotational movement and the x-ray beam sequentially pulsed giving rise to the different angular projections; while in 4th generation geometries, the different projections of the slice arise from the ring of stationary detectors placed around the slice with each detector taking sequential samples as the x-ray source rotates around the slice giving rise to a different angular projection. Compared to the 1st generation scanner, the 2nd, 3rd and 4th generation geometries detect a greater portion of the X-rays produced allowing for shorter scan times. The ease of the pure and continuous rotational movement upon the initiation of a scan of the 3rd and 4th generation machines compared to the start/stop motions associated with the earlier designs results in their being the current geometries marketed by most manufacturers.
In 3rd generation geometries the distance between samples in a projection is the detector-to-detector spacing which is coarse and limits the scanner's spatial resolution. In 1st, 2nd and 4th generation geometries the projections can be finely sampled and the basic limitation is the size of the detector aperture. Other factors which may limit the spatial resolution of a CT scanner are the cutoff frequency, f.sub.c, of the reconstruction algorithm or the Nyquist frequency, f.sub.p, of the image display consisting of square pixels of finite size. The algorithm cutoff frequency f.sub.c is usually (but not always) matched to the fundamental limitation of the scanner which is dictated by the smallest value of either the sampling Nyquist frequency (f.sub.s), the pixel display Nyquist frequency (f.sub.p), or the detector aperture cutoff frequency (f.sub.a).
Currently, detector apertures in 4th generation geometries are approximately four mm and anatomical detail much smaller than this is therefore not sharply defined. While the detector apertures can be stopped down, or made smaller to improve the scanner's resolution, when this is done a larger percentage of the X-rays emerging from the patient strike dead space between the detectors and are wasted, thereby reducing the x-ray dose efficiency discussed in more detail hereinafter. Obviously, those X-rays which strike detector dead space contribute no information to the resultant image, and their loss results in increased radiation dose to the patient.
At present, the major component of a CT Scanner's manufacturing cost is associated with the detectors, detector electronics, and computer processing system which in turn is dependent upon the number of detectors employed. Thus, when detector size is reduced to improve resolution and system performance, a greater number of detectors is required and the cost of the overall CT system is concomitantly increased such that a clear tradeoff between cost and resolution is evidenced by the present state of the art. In order to stay within reasonable commercial constraints, current 3rd and 4th generation scanners typically have between 500 and 1,200 detectors.
The concept of prepatient collimation to confine the x-ray beam to the region of interest to reduce the deleterious effects of large beams of radiation has been widely used in the medical x-ray industry since the turn of the century. It is currently employed in CT scanners to define the slice thickness and detector assemblies. Additional prepatient collimation designed to reduce the percentage of radiation striking the dead spaces between detectors has been introduced for 2nd generation geometries and marketed by EMI and Elscint. However, due to the geometrical unsharpness of the collimator at the detectors resulting from the finite size of the focal spot, such collimation schemes do not result in geometrical efficiencies of much greater than 65%. Ohio Nuclear also announced a 4th generation CT scanner, the 2020 .DELTA.-Scanner, which employed prepatient collimation. Their design consisted of a stationary ring of 720 4 mm detectors separated by 4 mm of dead spaces and the objective was to eliminate radiation from striking the 4 mm dead spaces. However, this design was dropped due to the finite size of the focal spot and the x-ray optics of the geometry making it impossible to confine the x-ray beam with prepatient collimation to the 4 mm detectors without a significant amount of radiation striking the dead spaces and also resulting in a significant loss in the x-ray flux striking the detectors.
Another prior art CT Scanner, the 7000 series by EMI Ltd., a variant 4th generation geometry consisting of a Nutating.RTM. rather than stationary array of 1,112 batch produced solid state detectors arranged as a small ring that wobbles as the x-ray source rotates around the patient, in which the detectors at 180.degree. from the x-ray source are translated in the opposite direction allowing the x-ray beam to strike the former. The circle defined by the rotation of the x-ray source is larger than that of the detector ring. Advantageously, the EMI 7000 CT Scanner enjoys improved system resolution as a result of the smaller detectors employed, and also reduced dead space due to close packing of the detectors.
The motivation for the design lies in the fact that fewer detectors are required than would be in the larger ring diameter of a conventional 4th generation design to achieve the same resolution. Nevertheless, while the EMI 7000 CT Scanner realizes improved system performance, the improvement is achieved at high cost due to the complicated mechanics of the Nutating.RTM. detector ring implementation. Furthermore, since the EMI 7000 Series approach employs a small detector ring, further advances in system performance are very much limited to detector technology, and more specifically, detector aperture size. Then, even if additional smaller detectors are used, the above-described tradeoff must nevertheless play the dominant role in arriving at a final system design.
Yet another CT Scanner under consideration by Pfizer involves the possibility of employing a conventional 4th generation design consisting of a stationary detector array comprised of a large number, as many as 2,400, small detectors essentially equivalent at least in their size and x-ray detection efficiency to those currently being used in the EMI 7000 Scanner. Once again, however, arbitrary increase in the number of detectors significantly complicates the detector electronics and related computer processing system, greatly increasing the cost of the total system.
Examples of additional prior art CT Scanners are found in U.S. Pat. Nos. 4,123,659 to Oliver, 4,101,768 to Lill, 4,097,747 to Kowalski, 4,097,744 to LeMay, 4,096,391 to Barnes, 4,096,389 to Ashe, 4,091,289 to LeMay, 4,075,491 to Boyd, 4,066,901 to Seppi, 4,048,505 to Hounsfield, 4,031,395 to LeMay, and 3,684,886 to Muehllegner.